Network Working Group F. Andreasen
Request for Comments: 5027 D. Wing
Updates: 3312 Cisco Systems
Category: Standards Track October 2007
Security Preconditions for
Session Description Protocol (SDP) Media Streams
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document defines a new security precondition for the Session
Description Protocol (SDP) precondition framework described in RFCs
3312 and 4032. A security precondition can be used to delay session
establishment or modification until media stream security for a
secure media stream has been negotiated successfully.
Table of Contents
1. Introduction ....................................................2
2. Notational Conventions ..........................................2
3. Security Precondition Definition ................................2
4. Examples ........................................................6
4.1. SDP Security Descriptions Example ..........................6
4.2. Key Management Extension for SDP Example ...................9
5. Security Considerations ........................................11
6. IANA Considerations ............................................13
7. Acknowledgements ...............................................13
8. Normative References ...........................................13
9. Informative References .........................................14
Andreasen & Wing Standards Track [Page 1]RFC 5027 Security Preconditions October 20071. Introduction
The concept of a Session Description Protocol (SDP) [RFC4566]
precondition is defined in [RFC3312] as updated by [RFC4032]. A
precondition is a condition that has to be satisfied for a given
media stream in order for session establishment or modification to
proceed. When a (mandatory) precondition is not met, session
progress is delayed until the precondition is satisfied or the
session establishment fails. For example, RFC 3312 defines the
Quality-of-Service precondition, which is used to ensure availability
of network resources prior to establishing (i.e., alerting) a call.
Media streams can either be provided in cleartext and with no
integrity protection, or some kind of media security can be applied,
e.g., confidentiality and/or message integrity. For example, the
Audio/Video profile of the Real-Time Transfer Protocol (RTP)
[RFC3551] is normally used without any security services whereas the
Secure Real-time Transport Protocol (SRTP) [SRTP] is always used with
security services. When media stream security is being negotiated,
e.g., using the mechanism defined in SDP Security Descriptions
[SDESC], both the offerer and the answerer [RFC3264] need to know the
cryptographic parameters being used for the media stream; the offerer
may provide multiple choices for the cryptographic parameters, or the
cryptographic parameters selected by the answerer may differ from
those of the offerer (e.g., the key used in one direction versus the
other). In such cases, to avoid media clipping, the offerer needs to
receive the answer prior to receiving any media packets from the
answerer. This can be achieved by using a security precondition,
which ensures the successful negotiation of media stream security
parameters for a secure media stream prior to session establishment
or modification.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Security Precondition Definition
The semantics for a security precondition are that the relevant
cryptographic parameters (cipher, key, etc.) for a secure media
stream are known to have been negotiated in the direction(s)
required. If the security precondition is used with a non-secure
media stream, the security precondition is by definition satisfied.
A secure media stream is here defined as a media stream that uses